1. Field of the Invention
The present invention relates to a magnetic recording medium having a thin ferromagnetic metal film and more particularly it relates to a magnetic recording medium having excellent anti-corrosive property.
2. Background Art
The demand for higher recording density in a magnetic recording system has recently increased. In this connection, a thin metal film type magnetic recording medium having a thin ferromagnetic metal film formed by a vacuum evaporation method or a sputtering method has been studied as a replacement for a conventional coating type magnetic recording medium for a recording layer.
In such a thin metal film type magnetic recording medium, there is formed a thin metal film composed of ferromagnetic metal alloy having high saturation magnetization and having a thickness of from 50 to 500 nm. Accordingly, the thickness of the magnetic layer is decreased one or two orders of magnitude below the thickness of the conventional coating type magnetic layer, thereby reducing the effects of demagnetization in the short wavelength region, and thus high density recording has been realized. Furthermore, in this thin metal film type, it is unnecessary to use organic solvents in a large amount unlike the coating type of magnetic film, and therefore, the manufacturing equipment can be simplified.
The types of ferromagnetic materials include iron, cobalt, nickel or alloys thereof. When cobalt is used, the desirable high coercive force can be relatively easily obtained due to its high monoaxial crystal magnetic anisotropy. For example, a cobalt nickel alloy thin film prepared as a magnetic tape by an inclined vapor deposition method has widely been studied.
In order to improve the environmental resistance and durability of such a thin metal film type magnetic recording medium, a method of nitrogenating the surface of a magnetic layer by an ion plating method has been disclosed in Japanese Patent Application (OPI) No. 33806/1975. (The term "OPI" used herein means an unexamined published application.) A method of providing a silicon nitride film by sputtering has been disclosed in Japanese Patent Application (OPI) No. 30304/1978. A method of forming a non-magnetic layer on a magnetic layer by exposing the magnetic layer to the discharge of nitrogen gas has been disclosed in Japanese Patent Application (OPI) No. 85403/1978. A method of providing a nitrogenated metal thin film on a thin magnetic metal film has been disclosed in Japanese Patent Application (OPI) No. 143111/1979. A thin magnetic film composed of iron nitride, or iron and iron nitride has been disclosed in European Patent No. 8328 and Japanese Patent Application (OPI) No. 87809/1984. Further, one of the inventors of the present invention with others suggested in their earlier application (Japanese Patent Application (OPI) No. 54023/1986) a magnetic recording medium comprising a non-magnetic support having provided thereon a thin magnetic film mainly comprised of iron nitride oxide. The composition of the thin magnetic film is shown by the following composition. EQU Fe.sub.1-x-y N.sub.x O.sub.y, 0.25.ltoreq.x+y&lt;0.60
A commonly assigned U.S. patent application, Ser. No. 133,829, filed Dec. 16, 1987 also discloses an iron nitride oxide magnetic film in which x+y.ltoreq.0.25.
On the other hand, a cobalt chromium alloy thin film prepared for a magnetic disk by a sputtering method has been extensively studied.
The magnetic layer of the above medium has an apparent mirror-like surface, but microscopically has a structure having metallic fine particles of a particle size from 10 to 100 nm. Therefore, when dew condenses on the magnetic layer or when the magnetic recording medium is exposed to an atmosphere containing gaseous sulfurous acid, the magnetic recording medium easily corrodes. Even if an extremely slight amount of corrosion is present on the surface of a magnetic recording medium, it comes off when the medium rubs against a magnetic head or other parts, thereby causing head clogging or dropouts.
In order to remove the above defect, an inorganic or organic protective layer on a thin metal film has been proposed.
However, if the above-described protective layer is provided, it must be thick to obtain good results, which is unfavorable in view of spacing loss. The above-described magnetic recording medium which is mainly comprised of iron nitride or iron nitride oxide has improved environmental resistance and improved anti-corrosive property, but still there is a defect in that saturation magnetization is not high enough to obtain high outputs.